© 2015 International Committee on Composite Materials. All rights reserved. A composite unit cell model is developed to estimate the effective transverse thermal conductivity of unidirectional green composites (K) based on a thermal-electrical analogy technique. The effective transverse thermal conductivity K is found to be expressed as a function of the geometrical ratio (, 0<<1) of lumen radius (rl) to fiber radius (rf), ratio () of thermal conductivities of fiber (Kf) to matrix (Km), and fiber volume fraction (Vf). The analytical results showed that, as the Vf increases, K changes in the following two styles: K increases when is smaller than a critical value c and K decreases when is larger than c. The critical value c depends only on the value of , in addition a smaller value of leads to a smaller value of c. Transverse thermal conductivity of unidirectional green composites is significantly affected by lumen size ratio . It is concluded that the green composites having better transverse thermal insulation characteristics can be designed by choosing the natural fibers with larger lumen.